The activation of proto-oncogenes is a general mechanism of carcinogenesis. Induction of breast cancer in mice by the retrovirus, mouse mammary tumor virus (MMTV), results from proviral insertion in the vicinity of one of a series of proto-oncogenes (the wnt-x series) and selective activation of these genes in mammary cells. We have identified at least three separate transcriptional control elements in the MMTV promoter: a tissue-selective enhancer at the 5 end of the LTR, an activity associated with the immediate proximal promoter, and a poorly defined internal region of the viral LTR that harbors a negative transcriptional element. The upstream tissue specificity element may form the basis of mammary-specific oncogene activation. This element can function to activate heterologous promoters in a cell-specific and hormone-independent fashion. We have characterized five proteins that act at this locus. Two proteins, mp5 and F3, contribute 80% of the activity of the enhancer. During the current reporting period, a factor specific to the F3 binding element was purified by affinity chromatography. The protein was micro-sequenced by the mass spectrometry technique, and identified as poly ADP-ribosylase, or PARP. Although this protein has been previously identified in conjunction with several transcription processes, its role remains unclear. It was recently proposed that the protein interacts specifically with the retinoic acid receptor (RAR), and also implicated in a matrix attachment region (MAR) binding complex. Expression of the MMTV promoter was characterized in differentiating mammary cells, using an in vitro model developed by Bissell and colleagues. In addition to the 5 LTR enhancer, sequences between -250 and -50 in the immediate proximal region of the MMTV promoter are required for efficient expression of the promoter in response to cellular differentiation. The -250/-50 region only functions in mammary cells when stably integrated into chromosomal sequences. This region is not active when introduced transiently into cells, indicating that some feature of template architecture is important for the function of this region. We propose that the organized chromatin environment present in replicating chromosomes is necessary for the functional activation of the promoter during cell differentiation. - mammary gland, Nuclear proteins, Oncogenes, Prostate cancer, Proto-oncogenes, Retroviruses, transcriptional control, Transformation, Transgenic Mice, Chromatin, Acetylation, - Neither Human Subjects nor Human Tissues